Nonaqueous electrolyte secondary cells such as lithium ion secondary batteries have been used in small-sized, portable electric and electronic devices (e.g. laptops, cellular phones, smartphones, tablet computers, ultrabooks) because they are high-voltage cells and have a high energy density, tend not to self-discharge, show less memory effect, and can be significantly lighter in weight. Nonaqueous electrolyte secondary cells are also being used in a wide range of applications, such as on-vehicle power sources for driving for automobiles or large-sized stationary power sources.
A technique for producing electrodes is a key factor in improving the energy density of nonaqueous electrolyte secondary cells. For example, electrodes of lithium ion secondary cells may be produced as follows. If a carbonaceous material such as coke or carbon is used as a negative electrode active material, the negative electrode may typically be prepared by powdering the carbonaceous material, dispersing the powdery material in a solvent together with a binder to prepare a negative electrode mixture, and applying the mixture to a negative electrode collector, followed by drying to remove the solvent and rolling the workpiece. In the present description, carbonaceous materials which merely absorb/release lithium ions are also referred to as active material. Similarly, the positive electrode may typically be produced by powdering a positive electrode active material (e.g., lithium-containing oxide), dispersing the powdery material in a solvent together with a conductive agent and a binder to prepare a positive electrode mixture, and applying the mixture to a positive electrode collector, followed by drying to remove the solvent and rolling the workpiece.
Thus, the electrodes are produced using an electrode mixture in the form of slurry obtained by dispersing, in an organic solvent, a powdery electrode material prepared from a positive electrode active material or a negative electrode active material and a binder.
Patent Literature 1 discloses an electrode for a nonaqueous cell, the electrode including a binder and an electrode active material. The binder is a fluorine-based polymeric copolymer mainly consisting of monomer units of vinylidene fluoride (A), hexafluoropropylene (B), and tetrafluoroethylene (C). The mol fractions XA, XB, and XC of the monomer units satisfy 0.3≦XA≦0.9, 0.03≦XB≦0.5, 0≦XC≦0.5, and 0.80≦XA+XB+XC≦1.
In the technique disclosed in Patent Literature 2, a lithium-containing oxide (e.g., LiCoO2) as a positive electrode active material and graphite as a conductive agent are mixed with polyvinylidene fluoride to prepare a positive electrode mixture, and the mixture is dispersed in N-methylpyrolidone to prepare a slurry. The slurry is then applied to aluminum foil as a positive electrode collector. Separately, a carbonaceous material as a negative electrode active material and polyvinylidene fluoride are mixed to prepare a negative electrode mixture, and the mixture is dispersed in N-methylpyrolidone to prepare a slurry. The slurry is then applied to copper foil as a negative electrode collector. The coated collectors are each dried and compression-molded with a roller press machine and thereby processed into electrode sheets.
Patent Literature 3 discloses a nonaqueous electrolyte secondary cell including a positive electrode formed of a positive electrode collector retaining a positive electrode mixture and/or a negative electrode formed of a negative electrode collector retaining a negative electrode mixture, and a nonaqueous electrolyte. The positive electrode mixture contains a binder for a nonaqueous electrolyte secondary cell, the binder comprising a binary copolymer consisting of 50 to 80 mol % of vinylidene fluoride and 20 to 50 mol % of tetrafluoroethylene, a positive electrode active material, and a conductive material. The negative electrode mixture contains the binder and a negative electrode active material.
Patent Literature 4 discloses a binder for a nonaqueous electrolyte secondary cell, the binder comprising a copolymer of vinylidene fluoride and tetrafluoroethylene and a specific PVdF. However, this literature fails to describe stability of the electrode mixture or adhesion of the electrodes, which are important characteristics for the electrodes.